The last men to walk on the moon blasted off its surface for a final time 40 years ago today.

After three days exploring the Taurus-Littrow lunar valley, Apollo 17 astronauts Eugene Cernan and Harrison Schmitt lit the engine on the upper (ascent) stage of their lunar module "Challenger" and launched off the surface at 5:55 p.m. EST (2255 GMT) on Dec. 14, 1972.

The last word spoken by a man on the moon (to date) — "Ignition" — was voiced by Schmitt, the first geologist and professional scientist to fly on a NASA mission. Cernan, as Apollo 17 commander, delivered the order to leave — "Let's get off" — just moments before.